Monocytes and macrophages have been shown to function in a myriad of host defense mechanisms. The spectrum of monocyte functions has become increasingly important and the heterogeneity among monocytes and macrophages has become more defined. It is not known whether the different monocyte functions are carried out by genetically restricted subsets, analogous to T cells and B cells of lymphocytes, or whether a multipotential single monocyte population is capable of functional differentiation in response to appropriated regulatory stimuli. The ability to study the functional potential of monocytes depends on the ability to isolate homogeneous populations and the ability to assess their functions in a reliable manner. We have recently isolated two subsets of human peripheral blood monocytes by elutriation centrifugation. While they share some properties, they differ with respect to others. We propose to improve the purification of monocyte subsets in peripheral blood and define their structural and functional characteristics. We will characterize each subset with respect to surface markers, histochemical staining, chemotaxis, phagocytosis, cytostatis, cytotoxicity, and production of mediators active in hematopoiesis and immune responsiveness. In analogous studies, we will define the peripheral blood monocyte subsets in the rat so that we can exploit this experimental animal system to define the origin, differentiation, maturation, and fate of these subsets. We will use in vitro culture systems to monitor maturation of function and structure of bone marrow precursors and peripheral blood monocytes. We will use in vivo assays with selected infusion of radiolabeled subsets to study maturation, tissue distribution, and function of these cells in inflammatory lesions and tumors. In order to render these assays technically feasible, and to provide for sequential testing of patient monocyte subsets in the future, we will continue our studies of optimum methods of cryopreserving each subset of monocytes. We hope to define the extent to which monocyte subsets are genetically restricted and their potential for functional differentiation.